When preparing wood fibres a separation of fibres from steam takes place in a fibre separation step where a cyclone or a centrifugal separator is used for this purpose. Preferably, the steam is recycled also from this step. The pressure in the system before the fibre separation is in most cases about 3–12 bar. The following process step is the drying of the fibre. This takes place at a lower pressure, sometimes as low as atmospheric pressure. In order to prevent the steam from escaping to the drying step at lower pressure a pressure tight feeding of the fibre from the separation step is required.
When treating mechanical pulp for paper manufacturing, this can easily be solved by a plug screw where the fed fibres are compressed and form a steam tight plug in the plug screw at the same time as it is further fed and subsequently may be fed to further treatment at a lower pressure.
On the contrary, when treating mechanical pulp for manufacturing of, for example, fibre board or MDF-technology, it is not possible to compress the fibres since lumps thus are formed, which cause problems at the drying of the fibre and at following manufacturing of end products. Instead a sluice feeder may be used in which the separated fibre fall down into a pocket in its rotor provided in the sluice feeder, which rotor is rotated and the fibre falls out in the lower portion of the sluice feeder where a lower pressure prevails without being compressed. The rotor and its pockets are sealed against the periphery of the sluice feeder.
A problem however is that steam escapes from the sluice feeder despite efforts to seal sluice feeders. Due to the design of the rotor with pockets it is complicated to achieve desired tightness and thus expensive. The annual costs for energy losses are great.